Ensure shutdown function does not raise exception on ENOTCONN

[francoisthire]

I stumbled across a case where the syscall shutdown of my application failed with ENOTCONN and the syscall to close did not occur resulting into a file descriptor leak.

By looking at the potential culprit, I found the shutdown function of this library.

The patch is inspired to what is done in lwt_io:

let close_socket fd =
  Lwt.finalize
    (fun () ->
       Lwt.catch
         (fun () ->
            Lwt_unix.shutdown fd Unix.SHUTDOWN_ALL;
            Lwt.return_unit)
         (function
           (* Occurs if the peer closes the connection first. *)
           | Unix.Unix_error (Unix.ENOTCONN, _, _) -> Lwt.return_unit
           | exn -> Lwt.reraise exn))
    (fun () ->
       Lwt_unix.close fd)

[anmonteiro]

we should be using Lwt.catch instead of try for Lwt functions.

[francoisthire]

I thought about using Lwt.catch, but the issue here is that it changes the type of the function. Indeed, the return type of shutdown is unit, and using Lwt.catch would turn it into unit Lwt.t. I am not familiar with this library, but it would look like a breaking change since the shutdown function is exposed. Let me know what is best.

[francoisthire]

I have pushed a commit which changes the type of shutdown. Let me know if we should keep it.